The present invention is directed to an oxide-based phosphor and a process including a oxide-based phosphor doped with praseodymium (Pr) and including at least one activator so that pumping with one photon of ultraviolet (UV) light produces two photons of visible light. The invention is particularly applicable to, but it not limited to, plasma display panels (PDP). These panels have gained increasing acceptance in display technology.
In the typical PDP, electrical discharges are produced in respective areas, i.e., pixels, addressed by electrical signals. The discharges occur within gases contained within the PDP, typically zenon and helium. The plasma discharge within these gases typically produces ultraviolet light, particularly vacuum ultraviolet (VUV) light with characteristic wavelengths at 147 nm and 172 nm which correspond to photon energies of 8.3 and 7.2 eV, respectively. This invisible light must be converted to visible light in order to produce an image on a PDP.
Typically, phosphors are employed for the conversion of the VUV light to visible light. A similar light energy conversion process is used in fluorescent lamps but is not desirable in PDPs because fluorescent lamps employ mercury which has delayed emission characteristics and adverse environmental considerations. Further, in a PDP, it is important that the phosphors produce colors to reproduce a color image. Many lamp phosphors have been successfully adapted to PDPs by enhancing the VUV light absorption properties and response times. However, the large energy difference between the pumping light and the emitted photons results in very low energy efficiency.
One approach to improving the energy efficiency is the use of a quantum cutter phosphor in which a high energy photon produces two lower energy photons. This process has been demonstrated in phosphors with fluoride-based lattices, such as YF3 and NaYF4. Quantum efficiencies for these quantum cutter phosphors exceeding 100% for visible light, i.e., light having wavelengths between 400 and 705 nm, have been reported. The quantum cutting phenomenon has been observed in an oxide phosphor lattice of SrAl12O19 doped with Pr. Nevertheless, the visible light quantum efficiency for this oxide-based lattice is relatively low because an energy transition producing a UV emission competes with the quantum cutting energy transitions that produce visible light. Other schemes employing quantum cutting in phosphors of LiGdF4 and GdF4 doped with europium (Eu) and producing two photons of the same color, i.e., red, with a conversion efficiency as high as 195% have been reported. The production of two photons with the same energy is a significant advantage over previous achievements with Pr-doped phosphors employing quantum cutting in which the two produced photons cover a wide spectral range, from infrared (IR) to UV.
In previous work with Pr-doped fluoride-based phosphor lattices, for example, LaF3 and NaYF4, the light emissions include a principal line having a wavelength at 407 nm and a second emission more clearly within the visible range and having a wavelength between 470 and 620 nm. An example is illustrated in FIG. 1. The energy levels and transitions involved in this process are schematically illustrated in FIG. 2. An electron of a Pr3+ ion is excited to a 4f5d band by the absorption of an incident pumping UV photon. The exciter electron non-radiatively decays to the 1S0 state. Then, the emitted light having a wavelength of 407 nm is produced from a 1S0-1I6 transition, followed by another non-radiative energy loss of the electron into the 3P0 level followed by a secondary radiative transition to various 3F and 3H levels. The latter transition produces visible luminescence at wavelengths between 420 nm and 650 nm. However, unless appropriate steps are taken to isolate some of the energy levels from the broad 4f5d band, the emission spectrum will be dominated by an intense UV emission. To avoid that result, a lattice with a weak crystalline field, such as a fluoride-based lattice, has been necessary to produce a useful quantum cutter phosphor. However, the fluoride-based lattices are unstable, preventing practical applications of those materials to PDPs and other applications, for example, lighting.
Oxide-based lattice phosphors are stable but have relatively strong crystalline fields. However, in oxide-based lattices with high coordination numbers, the crystalline field can be relatively weak. In addition, crystalline fields are expected to be reduced in strength in oxide-based lattices when doped with Pr, producing Pr3+ ions within the lattice. It is on that basis that an SrAl12O19 phosphor lattice doped with Pr exhibiting the quantum cutting phenomenon has been demonstrated. However, the quantum efficiency of this oxide-based lattice phosphor fails to achieve the efficiencies observed for fluoride-based lattices and, therefore, have not been satisfactory.
Accordingly, it is an objective of the invention to provide an oxide-based lattice phosphor that is stable, that has a relatively weak crystalline field due to the presence of Al, that has a high coordination number, that can be doped with Pr, and that exhibits the quantum cutting phenomenon in which two visible photons are produced in response to a single high energy UV pumping photon.
It is a further objective of the invention to produce such a phosphor having a quantum efficiency exceeding that previously observed with such oxide-based lattice phosphors and, preferably, which exceeds the quantum efficiency of fluoride-based lattice phosphors.
A method of producing two visible light photons from an oxide-based phosphor doped with praseodymium and including atoms of at least one activator in response to excitation with a single ultraviolet light photon according to the invention includes exciting the Pr of the oxide-based phosphor with a photon of ultraviolet light to excite an electron to an excited state, the excited electron falling to a lower energy state in a non-radiative transition and transferring energy to excite a first activator atom in the oxide-based phosphor, the first activator atom emitting a first photon of visible light, and the excited electron falling further to a lower energy state in a non-radiative transition, transferring energy to excite a second activator atom in the oxide-based phosphor, the second activator atom emitting a second photon of visible light.
The first and second photons have the same wavelength.
An oxide-based phosphor doped with praseodymium according to the invention includes atoms of at least one activator and emitting two visible light photons in response to excitation with a single ultraviolet light photon.
The activator atoms according to the invention are chosen from manganese, terbium, and europium.
A preferred oxide-based phosphor lattice is SrAl12O19.